1. Field of the Invention
The present invention relates to the processing of dispersions of inorganic fillers in polyester resins and, more particularly, to a method for reducing the dispersion viscosity of inorganic fillers, including alumina trihydrate, calcium carbonate, titanium dioxide, clay and silica, in polyester resins.
2. Description of the Art
Several inorganic filler materials, including hydrates such as alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O), are used in large quantities as flame retardant and smoke suppressive fillers for plastics, particularly for thermosetting polyesters. Such materials resist fire and heat and may contain water of hydration which is slowly released therefrom by dehydration in a fire situation. Increased loading of the fillers into the mixture necessarily increases the flame retardant and smoke suppressive qualities of the plastic material.
It is well known that the presence of alumina trihydrate, as is the case with other inorganic fillers or pigments, has the adverse effect of increasing the dispersion viscosity of the fillers in plastics, particularly polyester resins. Such increased viscosity is proportional to the amount of inorganic filler in the dispersion. Although loading the mixture with more than 50% filler or pigment may be desired, such a mixture exhibits an unworkable viscosity. Therefore, prior to the present invention, the loading had been limited by viscosity.
It is well known to those skilled in the art that the viscosity of many pigment-plastic mixtures can be increased or decreased by applying an appropriate coating to the pigment. For example, U.S. Pat. No. 3,780,156 teaches the addition of up to two percent of an acid modified lubricant, such as esters of ortho phosphoric acid, into a methyl methacrylate-alumina trihydrate mixture to reduce the viscosity of the mixture. However, the prior art is absent a teaching of the coating of the present invention applied to inorganic fillers before the filler is added to a polyester resin to reduce the viscosity of the resulting dispersion.
It is also known that hydrated fillers, such as alumina trihydrate, normally contain weakly bound surface water as well as tightly bound water of hydration. The concurrently filed patent application entitled "Method of Reducing the Viscosity of Polyester-Alumina Trihydrate Dispersions by Slight Dehydration of the Trihydrate" discloses a method of reducing the viscosity by dehydrating the alumina trihydrate to reduce the loss on ignition by 0.2 to 5.0 percent prior to adding the trihydrate to the polyester resin. In a preferred embodiment of the present invention, however, slight dehydration of the alumina trihydrate prior to coating in accordance with the present invention has the effect of reducing the viscosity of a polyester-alumina trihydrate dispersion even more than the reduction achieved through coating alone or through slight dehydration alone.
According to the present invention, the dispersion viscosity of inorganic fillers in polyester resins is reduced by at least 50 percent without affecting the flame retardant and smoke suppressive qualities of the filled polyester material.